Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity that may damage engine components. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions.
Various approaches may be provided for mitigating pre-ignition. In one approach, as shown by Glugla et al. in US 20120245827, in response to an indication of pre-ignition, a pre-ignition affected cylinder is enriched via multiple intake and/or compression stroke direct injections instead of a single direct injection. Fueling of remaining cylinders is then adjusted to maintain an exhaust air-fuel ratio of the engine at or around stoichiometry.
However the inventors herein have recognized potential issues with such an approach. As one example, while the charge cooling effect of the direct injection improves pre-ignition mitigation, it also generates more particulate matter emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. Similarly, the injected fuel may encounter less turbulence when flowing through the valves. Consequently, there may be pockets of richer than stoichiometric combustion that may generate soot locally, degrading exhaust emissions. Since the pre-ignition mitigating direct injection is a richer fuel injection, the propensity of degraded emissions may be higher. In addition, the enrichment can lead to increased fuel consumption. Further, if multiple cylinders are affected by pre-ignition concurrently, and each cylinder is enriched, fuel consumption may increase significantly.
The inventors herein have recognized that engines having central fuel injection (CFI) capabilities can be leveraged for improving pre-ignition mitigation. In such engine systems, fuel is injected into an intake manifold, upstream of engine cylinders and their corresponding intake ports, and downstream of an intake throttle. When fuel is injected into the engine intake, heat is transferred from the intake air and/or local engine components to the fuel and this heat transfer leads to atomization of a portion of the fuel, which results in cooling of the manifold, and thereby cooling of the manifold air charge. This cooling can be used to reduce the propensity of pre-ignition without necessitating enrichment. Thus in one example, the issues described above may be addressed by an engine method comprising: in response to an indication of pre-ignition, selectively increasing a first portion of fuel delivered to the engine via manifold injection (CFI injection) relative to a second portion of fuel delivered to the engine via one or more of port and direct injection while maintaining an air-fuel ratio from before the indication of pre-ignition. In this way, by injecting a portion of the total amount of fuel via central fuel injection, manifold charge cooling may be increased, thereby reducing possibility of pre-ignition with reduced reliance on cylinder enrichment.
As one example, in response to an indication of pre-ignition, on an engine cycle immediately subsequent to the engine cycle where the pre-ignition was detected, a portion of fuel delivered to the intake manifold via a central fuel injector may be increased to increase the manifold charge cooling effect. For example, a pulse width of a central fuel injector may be increased, while maintaining manifold injection at or below a threshold limit based on engine speed and load conditions to ensure that all the fuel vaporizes and that no puddling of fuel occurs inside the manifold. In addition, port injection of fuel to the pre-ignition affected cylinder may be timed to occur during an open intake valve event to increase the charge cooling effect of the port injected fuel. While increasing the manifold injection of fuel, a stoichiometric air-fuel ratio may be maintained by correspondingly adjusting the amount of fuel delivered to each cylinder via port injection and/or direct injection. In addition, a pulse-width of fuel delivered via port injection and/or direct injection may be adjusted to account for maldistribution of fuel between downstream cylinders due to the upstream manifold fuel injection. The split ratio between the central fuel injector, the port fuel injector, and the direct injector may be adjusted to achieve a highest level of manifold charge cooling and in-cylinder charge cooling. As such, the relative increase in manifold injection may be higher than the relative increase in port injection for a number of enrichment cycles following the indication of pre-ignition until a pre-ignition propensity falls. If there is a further indication of pre-ignition following the increase in manifold injection, cylinder enrichment via one or more of port and direct injection may be used to mitigate further pre-ignition.
The technical effect of increasing the ratio of fuel delivered to an engine via manifold injection responsive to an indication of pre-ignition is that the charge cooling properties of a manifold injection may be leveraged for pre-ignition mitigation, reducing the need for cylinder enrichment. By enabling a stoichiometric air-fuel ratio to be maintained for a longer duration of engine operation following a pre-ignition event, fuel efficiency may be improved. In addition, even if manifold injection is increased responsive to pre-ignition incidence in a given cylinder, due to the location of manifold injection being upstream of all cylinders, the charge cooling properties of the manifold injection may be leveraged in all the downstream cylinders, reducing the propensity of pre-ignition in all cylinders in addition to the pre-ignition affected cylinder. By reducing the need for cylinder enrichment, fuel economy is improved. Further, increased particulate matter emissions from cylinder enrichment via direct injection is averted.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.